The discovery of glass fibers which can conduct light with very small losses, of low-cost solid-state diode lasers, and of techniques for masking and etching surfaces by an electron beam to form structures of micron size, has created a demand for solid-state optical waveguide modulators. An optical waveguide modulator alters the frequency, intensity, or polarization of a beam of light traveling in a waveguide. This can be accomplished by guiding the light through a specific material, which may be a single cyrstal, and generating acoustic waves in this material.
Crystals which are useful in optical waveguide modulators are those which are highly efficient at modulating light with an acoustic wave. A measure of this efficiency is given by the opto-acoustic figure of merit, M.sub.2, which is equal to EQU n.sup.6 p.sup.2 /.rho.v.sup.3
Where n is the refractive index, p is the photo-elastic coefficient, .rho. is the density, and v is the acoustic velocity. Large values of figure of merit are obtained in materials with a large refractive index and a low acoustic velocity.
In addition to a high opto-acoustic figure of merit, a good crystal should exhibit a low attenuation of high frequency sound waves so that the crystal can be used to modulate a wide band of light frequencies. The modulation of a wide band of light frequencies requires the propagation of acoustic waves at high frequencies, but in most materials high frequency acoustic waves are absorbed after traveling only a very small distance. Therefore, a material which can be made in very thin single crystal layers can be used to modulate a wider band of light frequencies since high frequency acoustic waves cross the cyrstal before being absorbed. However, it is very difficult to make a very thin single crystal layer of most suitable materials by sputtering, vacuum evaporation, or other available techniques since usually a polycrystalline layer is produced instead of a single crystal. The polycrystalline layers produced this way are optically very lossy. Cleavage of the material into very thin plates is not possible with most otherwise-suitable materials since their crystal structures do not permit such cleavage to occur.
Finally, the crystal should have good optical quality and a transparency range which includes the wavelengths of common solid-state light diodes.
A good summary of thin film waveguide devices can be found in an article by P. K. Tien in the April, 1974 issue of Scientific American, titled "Integrated Optics."